This hadith sums up the qualities of the people worthy of Jannah.
Firstly, just as the hearts of birds are free of jealousy, the hearts of the people deserving Jannah are free of jealousy, rancor, and envy.
Secondly, just as birds are always alert (cautious of any danger), the hearts of the people of Jannah are also alert and wakeful. They worry about their Akhirah, they are alert to not stray off the Straight Path, they are vigilant against the whispering of Satan, and their hearts are active in the remembrance of Allah Most High.
Thirdly, just as birds trust in Allah for their food, the people of Jannah have complete trust in Allah Most High. Birds don’t store food for the next day, nor indeed for any contingency. They go out hungry from their nests every morning but return with full stomachs every evening. The people worthy of Jannah similarly have complete trust (Tawakkul) in Allah regarding not only their sustenance, but, their very existence.
May my heart become like the heart of a bird. Ameen.